This invention relates to Doppler-type ultrasonic flowmeters and more particularly to the compensation of the flowmeter for variations in temperature.
In ultrasonic flowmeters of the Doppler type, a wave of ultrasonic energy is directed into the fluid whose flow is to be measured substantially along the axis of fluid flow. The flow is determined by the change in frequency of the ultrasonic energy as it is reflected from the flowing fluid. In order for there to be reflections from the fluid, it is necessary that the fluid contain reflectors which may be in the form of particles, sediment, bubbles or other masses within the fluid which provide different acoustic impedance from that of the fluid.
The relationship between the Doppler frequency and the velocity of the fluid is given by the following equation: EQU v=-f.sub.D C.sub.F /2f.sub.S Sin .theta..sub.F ( 1)
where
f.sub.S is the frequency of the ultrasonic wave from the transmitter PA1 C.sub.F is the speed of sound in the fluid PA1 f.sub.D is the Doppler frequency, and PA1 .theta..sub.F is the complement of the angle of attack (the angle between the direction of movement of the fluid and the ultrasonic beam path)
As it is well known that the speed of sound in a medium changes with the temperature of the medium, the relationship between the velocity of the fluid and the Doppler frequency as expressed in equation (1) will vary with the temperature of the fluid. Most of the prior art applications of Doppler ultrasonic flowmeters is in the measurement of blood flow and the problem of large temperature variations is not encountered. U.S. Pat. No. 3,741,014, directed to an ultrasonic flowmeter, identifies the problem of inaccuracy present in conventional ultrasonic Doppler flowmeters due to temperature changes of the medium. In order to overcome the problem, that patent proposes that there be inserted in front of the transmitter and the receiver a prism lens to automatically change the direction of the ultrasonic beam of the transmitter and receiver in response to temperature changes of the fluid.